1. Field of the Invention
The present invention relates to a superconducting device and a method for manufacturing the same, and more specifically to a superconducting device having an extremely thin superconducting channel formed of oxide superconductor material, and a method for manufacturing the same.
2. Description of Related Art
Devices which utilize superconducting phenomena operate at high speed with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature.
Josephson device is one of well-known superconducting devices. However, since Josephson device is a two-terminal device, a logic gate utilizing Josephson devices becomes complicated. Therefore, three-terminal superconducting devices are more practical.
Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). The first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both side of the semiconductor channel. A portion of the semiconductor layer between :the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulator layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode.
A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to a superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a low power consumption.
The second type of the super-FET includes a substrate, a channel of a superconductor formed on the substrate, a superconducting source region and a superconducting drain region positioned at the both sides of the superconducting channel, a source electrode and a drain electrode respectively positioned on the superconducting source region and the superconducting drain region, a gate electrode on the superconducting channel, and a gate insulating layer between the superconducting channel and the gate electrode. Superconducting current flowing through the superconducting channel of the super-FET is controlled by a voltage applied to a gate formed above the superconducting channel.
Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating output signal from input one and of having a well defined gain.
However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few ten nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short electrode distance mentioned above.
On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to product the first type of the super-FET is not necessary to product this type of super-FET.
In order to obtain a complete ON/OFF operation, both of the superconducting channel and the gate insulating layer should have an extremely thin thickness. For example, the superconducting channel formed of an oxide superconductor material should have a thickness of less than five nanometers, namely about four or five unit-cells and the gate insulating layer should have a thickness more than ten nanometers which is sufficient to prevent a tunnel current.
In the prior art, the extremely thin superconducting channel of the oxide superconductor has been usually formed directly on a single crystalline insulator substrate such as MgO (100) substrate. In this case, constituent elements of the substrate are diffused into the superconducting channel while the superconducting channel is formed on the substrate. Since the superconducting channel has an extremely thin thickness, the effect of the diffusant is not negligible. Therefore, properties of the superconducting channel is spoiled so that the super-FET can not have high performance.
Even if the effect is negligible, one or two unit-cells of the bottom side of the superconducting channel formed of the oxide superconductor may not behave as a superconductor. Therefore, the substantial thickness of the superconducting channel becomes thinner than the physical thickness, which in turn the superconducting current through the superconducting channel is reduced than expected.
The non-superconducting regions are considered to be generated by a bared Cu--O plane at the bottom surface of the oxide superconductor layer which constitutes the superconducting channel.
In addition, since the extremely thin superconducting channel is connected to the relatively thick superconducting source region and the superconducting drain region at their lower portions and is far from the source electrode positioned on the superconducting source region and the drain electrodes positioned on the superconducting drain region, the superconducting current flowing through the superconducting channel spreads in the superconducting source region and the superconducting drain region. Therefore, the superconducting current does not flow into nor flow from the superconducting channel, efficiently.